I was wondering.. I wanted to burn multiple mp3 cds. But when i put my .m3u in and the filesize is larger than the disk, it asks for a bigger medium. Why can't it just burn my music on the go? like if it doesnt fit one the first disc, it will go on the 2nd disc, and then the third etc until all files are burned..

Because the information needed for the content to be played properly is on the first disc and is only burned once. That's not a Nero limitation. It's in accordance with the CD Digital Audio standard. I'm not competent enough to explain it further.